The role of event beds in the preservation of organic carbon in fine-grained sediments: Analyses of the sedimentological processes operating during deposition of the Whitby Mudstone Formation (Toarcian, Lower Jurassic) preserved in northeast England
The role of event beds in the preservation of organic carbon in fine-grained sediments: Analyses of the sedimentological processes operating during deposition of the Whitby Mudstone Formation (Toarcian, Lower Jurassic) preserved in northeast England
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DOI：
10.1016/j.marpetgeo.2012.01.001
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发表时间：
2012-08
影响因子：
4.2
通讯作者：
S. Ghadeer;J. Macquaker
中科院分区：
文献类型：
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作者：
S. Ghadeer;J. Macquaker
地质学家最近观察到，烃源岩通常包含各种各样的微组构，包括小型的波纹和洞穴。这些织物的存在挑战的假设，即持续的底层水缺氧和低能量的条件是必要的先决条件，在这些单位加强有机碳保存。为了加深我们对这些地层中有机碳保存的沉积和成岩过程的理解，利用光学、电子光学和地球化学方法对下侏罗统惠特比泥岩组进行了研究。确定了四种薄层泥岩岩相：（a）含粉砂、富粘土泥岩;（B）富粘土泥岩;（c）含粘土、钙质微型浮游生物和有机碳泥岩;（d）富水泥泥岩。内部独立的床通常是尖锐的基础上，正常分级。它们要么是层压的，要么是部分生物扰动的，要么是均质的。在沉积结构保存，丸，淤泥滞后，涟漪层压，贝壳路面，波浪增强沉积物重力流的流体泥浆，和有机-矿物聚集体已被观察到。存在高达14.2%的TOC的单位，是尖锐的基础和正常分级表明，质量流，可能产生的风暴可能会影响这些泥岩沉积过程中的沉积。这些单元中丰富的有机-矿物组构表明，至少有一部分有机碳是作为海洋雪聚集体输送到海底的，这可能是在浮游植物大量繁殖后在水柱中形成的。床顶的均匀化表明，在沉积物输送事件之间，底栖动物能够在海底定居，这意味着在这些间隔期间，底部沃茨至少部分充氧。这些组构表明，在沉积过程中的惠特比泥岩形成，有机碳被优先保存在高初级有机生产的区域之下。尖锐的基础上，梯度床和有机-矿物聚集体之间的密切联系表明，大量的有机碳被交付到海底的episodically，在沉积物-水界面的条件是episodically动态。在这里，有机碳很可能已被优先保存，因为沉积事件的频率足够高，以尽量减少氧化剂扩散到沉积物和有机碳矿化。虽然底层水缺氧可能是在浮游植物大量繁殖期间发展起来的，但不太可能是持久性的，因为目前的微组构表明，有足够的时间和氧气可供底栖动物破坏沉积事件之间的沉积物。
Geologists have recently observed that source rocks commonly contain a wide variety of microfabrics, including diminutive ripples and burrows. The existence of these fabrics challenges the assumption that persistent bottom-water anoxia and low-energy conditions are necessary prerequisites for enhanced organic carbon preservation in these units. In order to enhance our understanding of the depositional and diagenetic processes responsible for organic carbon preservation in these strata, the lower Jurassic Whitby Mudstone Formation, which contains mudstones with up to 14.2% total organic carbon, has been investigated using optical, electron optical and geochemical methods. Four thinly-bedded mudstone lithofacies were identified: (a) silt-bearing, clay-rich mudstones; (b) clay-rich mudstones; (c) clay-, calcareous nannoplankton-, and organic carbon-bearing mudstones; and (d) cement-rich mudstones. Internally individual beds are typically sharp-based and, normally-graded. They are either laminated, partially bioturbated or homogenized. Where depositional fabrics are preserved, pellets, silt lags, ripple lamination, shell pavements, wave enhanced sediment gravity flows of fluid mud, and organo-minerallic aggregates have been observed. The presence of up to 14.2% TOC in units that are sharp based and normally-graded suggests that mass flows, possibly generated by storms likely influenced sedimentation during deposition of these mudstones. The abundant organo-minerallic fabrics in these units indicates that at least some of the organic carbon was delivered to the seafloor as marine snow aggregates, that likely formed in the water column following phytoplankton blooms. Homogenization of bed tops indicates that between sediment delivery episodes, an infauna was able to colonize the seafloor, implying that the bottom waters were at least partly oxygenated during these intervals. These fabrics suggest that during deposition of the Whitby Mudstone Formation, organic carbon was being preferentially preserved below regions of high primary organic production. The close association between sharp-based, graded beds and organo-minerallic aggregates indicates that large volumes of organic carbon were delivered episodically to the seafloor, and that conditions at the sediment-water interface were episodically dynamic. Here organic carbon is likely to have been preferentially preserved because the frequency of depositional events was sufficiently high to minimize both oxidant diffusion into the sediment and organic carbon mineralization. While bottom water anoxia may have developed during the phytoplankton blooms, it was unlikely to have been persistent, because the microfabrics present indicate that there was sufficient time and oxygen available for an infauna to disrupt the sediment between deposition events.